Quiz 1

Which choice best describes the age distribution of oceanic crust? a) oceanic crust is oldest at the ridge and gets younger with increasing distance from the ridge b) oceanic crust is older on one side of the ridge and younger on the other c) oceanic crust is youngest at the ridge and gets older with increasing distance from the ridge d) there is no orderly distribution to the age of oceanic crust

c) oceanic crust is youngest at the ridge and gets older with increasing distance from the ridge

Identify the three basic types of plate boundaries and describe the major geologic and tectonic processes occurring at each.

Divergent boundaries: plates move away from each other and the lithosphere is created Transform boundaries: plates slide past one another and the lithosphere is neither created or destroyed Convergent boundaries: plates move toward one another and the lithosphere is destroyed two parts of the convergent boundary: subduction zone and he continental collision zone New oceanic crust made by seafloor spreading at divergent boundaries, old oceanic crust recycled into mantle by subduction at convergent boundaries; both volcanism and earthquakes at both divergent and convergent boundaries, earthquakes only and no volcanoes at transform boundaries because no magma is made in this setting.

Describe the Earth's internal structure in terms of both chemical composition and physical properties.

The earth is divided into three layers, a crust, a mantle and a core, that differ in their chemical composition. The crust is the thin outer layer, consisting primarily of oxygen, silicon and aluminum. It is further subdivided into continental and oceanic crust. The oceanic crust is thinner and denser, and is similar in composition to basalt (Si, O, Ca, Mg, and Fe). The continental crust is thicker and less dense, and is similar to granite in composition (Si, O, Al, K, and Na). The mantle is made of magnesium, iron and silicon. The core is almost exclusively iron and nickel. The outer core is liquid iron and the inner core is solid iron. The mantle and crust are further divided into the lithosphere, asthenosphere and mesosphere, depending on their physical properties, namely how close the material is to its melting point. The lithosphere is cold and rigid and includes the curst and uppermost part of the mantle where rocks are far below their melting point. Further down in the mantle is the asthenosphere where rocks are weak and can flow because they are close to their melting point. The deepest part of the mantle is the mesosphere where rocks are gain strong and below their melting point due to the high pressure. softer and more pliable asthenosphere made of the remainder of the upper mantle where rock is closer to melting point

A hotspot is: a) an area of volcanic activity with a deep magma source b) an area of unusually strong earthquakes c) a strongly magnetic section of sea floor d) a place where sea floor is destroyed

a) an area of volcanic activity with a deep magma source

When two oceanic plates collide: a) the older plate will subduct b) the thinner plate will subduct c) the weaker plate will subduct d) neither plate will subduct, instead a mountain range is formed

a) the older plate will subduct

Buoyant materials are added to the edge of continents by: a) erosion and sedimentation b) subduction of the surrounding oceanic crust c) continental rifting d) seafloor spreading

b) subduction of the surrounding oceanic crust

The locations and ages of islands in the Hawaiian Island chain can be used to show: a) the direction of movement of the North American Plate b) the rate of movement of the Pacific Plate c) the size of earthquakes to be expected in the Pacific Ocean d) all of the above

b) the rate of movement of the Pacific Plate The Hawaiian Islands have been built by the passage of the Pacific plate over a mantle hotspot. The location and ages of the islands show the direction of movement of the Pacific plate as it passed over the hotspot. This is called a hot spot track. Because the Hawaiian islands are on the Pacific Plate they tell us nothing about the direction or rate of movement of the North American plate - the North American plate moves independently of the Pacific plate. They also don't tell us anything about the size of earthquakes to be expected in the Pacific Ocean - while passage of the plate over the hotspot, and subsidence as it moves off the hot spot results in earthquake activity, the largest earthquakes occur on the subduction zones that ring the Pacific basin.

Oceanic crust is produced by seafloor spreading. How is continental crust produced? a) by accretion of bouyant materials such as oceanic sediments at subduction zones b) by partial melting in the mantle at subduction zones, resulting in magmas enriched in silica c) both of the above d) actually, continental crust is neither produced or destroyed

c) both of the above Continental crust is produced at subduction zones as buoyant materials on the subducting plate are accreted to the leading edge of the overriding plate, while the remainder of the plate (due to its higher density) is subducted into the mantle. Introduction of water into the mantle by the subducting plate causes melting in the upper mantle producing magmas that are richer in silica and more like the continental crust than they are the mantle or the subducting plate. These magmas may erupt at the surface but most crystallize within the crust of the overriding plate as intrusion. Either way, more material is added to the continental plate. Both terrane accretion and partial melting makes continental crust by separating out buoyant materials from the subducting plate.

At a passive continental margin the primary geologic process occurring is: a) seafloor spreading b) subduction c) sedimentation d) volcanism

c) sedimentation Passive continental margins are places where the crust changes from thick continental crust to thin oceanic crust, but functioning as one plate- moving at the same direction and speed - and as such do not coincide with a plate boundary. When that area of continental crust first started breaking up by continental rifting, it was at a new divergent boundary, but is now far from the plate boundary due to addition of new oceanic crust to the edge of each plate by seafloor spreading- with the plate boundary remaining at the ocean ridge. As the edge of the continental crust gets further and further from the divergent plate boundary where hot mantle rock is rising, the continental crust cools and subsides (having been heated and elevated during the rifting stage). Since the crust was also thinned by the rifting process, its not as thick as it once was and drops below sea level as it cools, forming a broad continental shelf. This provides a place for large amounts of sediment to accumulate - this sedimentation is the primary geologic process occurring in a passive margin setting.

How did seafloor spreading revive Alfred Wegener's ideas about continental drift? a) sea floor spreading relied on new fossil evidence documenting the former joining of the continents b) sea floor spreading provided an explanation for the consumption of oceanic crust at plate boundaries c) the topography of the ocean floor demonstrated that mountain ranges on either side of the Atlantic Ocean were formerly joined d) sea floor spreading provided a viable mechanism for moving the continents e) sea floor spreading provided proof of plate tectonics

d) seafloor spreading provided a viable mechanism for moving the continents

Subduction zones were discovered when: a) deep trenches on the ocean floor were mapped b) when no old oceanic crust could be found anywhere c) when it was noticed that magnetic reversals occur periodically d) when it was noticed that deep earthquakes are found only near ocean trenches, and that earthquakes increase in depth with distance from the trench

d) when it was noticed that deep earthquakes are found only near ocean trenches, and that earthquakes increase in depth with distance from the trench Remember from Lecture 3 that earthquakes are not randomly distributed across the globe. Earthquakes are concentrated along plate boundaries. In particular, deep earthquakes (earthquakes that originate below the crust) are found only in association with ocean trenches. Further, earthquakes get deeper with distance from the trench, but only to one side of the trench (the "inland" side). These earthquakes mark the descent of cold, rigid oceanic lithosphere into the asthenosphere. Normally earthquakes are not possible in the asthenosphere because the asthenosphere is a soft solid close to its melting temperature. But where subduction occurs, cold, rigid oceanic lithosphere (the subducting plate) is sinking into the asthenosphere. The subducting plate remains cold enough and rigid enough to produce earthquakes to depths up to 700 km. This is how the process of subduction, and the places it occurs, was discovered - by the relationship of deep earthquakes to trenches.

In what ways does continental crust differ from oceanic crust? a) continental crust is thicker than oceanic crust b) continental crust is less dense and more bouyant than oceanic crust c) continental crust is richer in light elements like N and K than oceanic crust d) continental crust is poorer in dense elements such as Fe and Mg than oceanic crust e) all of the above

e) all of the above Continental crust differs from oceanic crust in all of the following ways: its thicker, less dense, more buoyant, richer in light elements like Na, K, and Si, and poorer in dense elements like Fe and Mg relative to oceanic crust